Dear Muhammad

Using waterphobic or water vapor phobic surfaces could be a solution. This is in the area of surface nano engineering. Search the keyword for further information.

Regards.

Muhammad,

I do not know what is the pressure inside tubes. If the pressure is relatively low you can leak the vapor by using chimneys. This may be a possible solution.

If this solution works with you then please tell me.

Best wishes

It sounds like the problem fundamentally arises from having vapour and liquid co-exist inside a single transparent vessel. Rather than fixing the symptom of removing condensation from an unwetted surface, I'd suggest removing the problem entirely by having the whole heating vessel filled with liquid, so the inside of the transparent surface is in constant contact with it.

This is very similar to the problem of "circulation" faced in conventional high-power steam boilers and water-cooled nuclear reactors; the liquid phase of the working fluid has a much higher capacity to absorb heat than the gas phase, so if the heat exchange surface is wetted it can run at a very high rate of heat flux without overheating and melting, but if the liquid touching it then boils and is not immediately removed, the vapour phase it flashes into can now remove far less heat, and the now-dry surface promptly overheats. (in nuclear reactors this also directly affects the fission process itself due to the different moderating effects of the liquid and vapour, modelled by the "void coefficient") This is solved by ensuring constant "circulation" of the working fluid to carry generated gas away from the heating surfaces and replace it with liquid, to maintain a constant rate of cooling. Circulation may be achieved by thermal convection, mechanical pumps, Giffard eductors (more commonly known as "steam injectors"), physical cycling of the exchange surface itself (in the manner of rotary heat exchangers/regenerators) or anything else you can think of.

The problem in your particular case is not a lack of heat flow due to loss of thermal conductivity, but rather a loss of heat flow due to obstruction and/or diffusion of thermal radiation, but it would be amenable to the same solution: constant wetting/submersion of the surface by continuous removal of the vapour phase, aka circulation.

I would suggest a good starting point would be researching the approach used in Foster-Wheeler D-type marine boilers and other high-pressure steam generators, which use the natural convection method to achieve circulation, by having separate drums at different heights for water and steam, and two sets of tubes between them; heat exchange tubes which are kept constantly wetted as the liquid in them is heated, expands and rises to the steam drum, and insulated downcomer tubes to return liquid from the steam drum to the water drum.

Another approach would be to look at liquid metal-cooled reactors or pressurised water reactors, which maintain the pressure of the coolant inside the reactor itself such that the coolant remains liquid throughout, and can then either boil a secondary circuit of lower-pressure coolant via a heat exchanger or theoretically be passed through an expansion device and allowed to vapourise itself, but only after it leaves the heating vessel (the latter is never done in practice in nuclear plants because the primary coolant is hellishly radioactive and you don't want radioactive gas leaking through turbine labyrinth seals, but it's obviously not an issue for a solar collector!)

If you really wanted to get fancy, you could try varying the solar absorptivity of the two-phase fluid passing through the collector by adjusting its pressure to regulate the ratio of liquid to vapour.